62 Reproduction and Life-Cycles 



nucleus. After differentiation of cilia, the larva begins to rotate within 

 the brood pouch. Increasingly vigorous contractions of the parent finally 

 expel the larva (196). The development of sporoblasts in various Cnido- 

 sporidea (Chapter VI) also may be considered a form of internal budding. 



In certain other Protozoa, budding may follow a series of nuclear di- 

 visions. Tritrichomonas aiigiista, although usually reproducing by fission, 

 sometimes develops into a somatella which undergoes budding (Fig. 2. 

 14, F, G). A similar process in Colacium vesiciilosum (Fig. 2. 14, A, B) 

 involves a multinucleate stage without flagella or reservoirs. These struc- 

 tures appear in each bud before it is separated from the parental somatella 

 (134). 



Schizogony, involving the production of several to many buds more 

 or less simultaneously, is characteristic of certain Protozoa. This process 

 is especially efficient in many Sporozoa in which the plasmodium (Fig. 2. 

 1, A) often contains many nuclei before schizogony (Fig. 2. 1, B). A 

 schizont of Eimeria bovis, for example, may produce as many as 170,000 

 merozoites (108). 



NUCLEAR DIVISION 



Although mitosis has been reported in most species which have 

 been studied carefully, the small size of many nuclei has made it difficult 

 to interpret the structure of chromosomes in early mitosis and in the 

 interphase. The interphase chromatin of Cryptomonadida (115) and 

 ZellerieUa elUptica (Fig. 1. 19, G) has been described as fine granules dis- 

 persed on a network; that of Pelomyxa carolinensis (Fig. 2. 7, A), as 

 Feulgen-positive granules and short filaments. The Feulgen-positive inter- 

 phase chromatin of Euglenida, according to different reports (115), ranges 

 from periendosomal granules to a continuous spireme which in optical 

 section simulates separate gianules. Actually, it has been impossible to 

 find stages suggesting an achromatic network containing chromatin gran- 

 ules in some of the Euglenida and Dinoflagellida. Instead, beaded chromo- 

 somes seem to persist through vegetative stages. In general, however, 

 chromosomes of the later prophases seem to develop from some sort of a 

 "reticulum" and, in such favorable material as Pamphagus hyalinus (17), 

 the process has been traced in living material. In certain species of Enta- 

 moeba (Fig. 1. 20, F) and in Naegleria gruberi (207), the chromosomes 

 develop from a finely granular or reticular zone of Feulgen-positive mate- 

 rial around the endosome. The persisting "peripheral chromatin" gran- 

 ules, apparently adherent to the nuclear membrane in Entamoeba, may 

 give rise to chromosome-like bodies perhaps analogous to the nucleoli of 

 ZellerieUa (58). Interpretations are even more difficult in Endamoeba 

 blattae because the interphase nucleus is Feulgen-negative, although 

 Feulgen-positive chromosomes appear in mitosis (177). 



The origin of chromosomes from an endosome or a karyosome, in 



